1. Field of the Invention
The present invention relates to a flip-chip-mounted surface acoustic wave device capable of downsizing.
2. Description of the Related Art
In recent years, downsizing and upgrading of functions are rapidly advancing for wireless devices such as mobile phones. Filters are used for the high-frequency circuits of those devices and play an important role. Generally, such filters are often configured using SAW (Surface Acoustic Wave) devices in order to downsize those circuits.
For example, in a wireless device such as a PCS (Personal Communication System) mobile phone, a transmission (Tx) filter and a reception (Rx) filter constituting the function of an antenna duplexer having a role of isolating a transmission (Tx: 1,850 to 1,910 MHz) signal and a reception (Rx: 1,930 to 1,990 MHz) signal from each other, are constituted of SAW apparatuses.
The above transmission (Tx) filter and the reception (Rx) filter are band-pass filters and are constituted of, for example, as described in Japanese Patent Application Laid-Open Publication No. 2004-194269, a combination of multiple (double) -mode SAW filters (DMS) having a plurality of (two) resonance modes by utilizing a plurality of (two) standing waves generated between reflecting electrodes.
As a double-mode SAW filter, an IDT (interdigital transducer) pattern as described in Japanese Patent Application Laid-Open Publication No. 2004-194269 as an example and shown in FIG. 1 is formed on a piezoelectric element (piezoelectric substrate) not shown such as LiTaO3, LiNbO3, Li2B4O7 or crystal.
In the example shown in FIG. 1, the IDT pattern formed on a piezoelectric element has 1 (one) input (output) IDT1 and 2 (two) outputs (inputs) IDT2a and 2b arranged alternately, and grating-type reflectors 3a and 3b positioned in the propagation direction of the surface acoustic wave on both sides sandwiching those input and outputs.
A characteristic of the IDT pattern shown in FIG. 1 is that the terminal IN (OUT) of the input (output) IDT1 and the terminal OUT (IN) connected commonly with the 2 (two) outputs (inputs) IDT2a and 2b are arranged on the same side.
Employing the IDT pattern shown in FIG. 1 as a unit, the filter is constituted by connecting a plurality of IDT patterns in parallel or in series (in a cascade) in response to a desired filter performance that is aimed at.
FIG. 2 is a plan view of a double-mode SAW filter (DMS) which includes 3 (three) parallel-cascade-connected units, employing the IDT pattern shown in FIG. 1 as a unit, and 3 (three) unit IDT patterns are connected in parallel cascades on a piezoelectric element 10.
FIG. 3 is a sectional view of a surface acoustic wave (SAW) device formed with a double-mode SAW filter (DMS) having the IDT pattern shown in FIG. 2, housed in a package. In this surface acoustic wave (SAW) device, a double-mode SAW filter (DMS) is pasted on the internal bottom surface of a ceramic package 11 with conductive paste 12. As the double-mode SAW filter (DMS), the IDT patterns are formed on the piezoelectric element 10 and the input terminal IN and the output terminal OUT and GND are connected with terminals on the ceramic package 11 side with bonding wires 13. A metal cap 14 is pasted covering the interior of the ceramic package 11 leaving a spatial spacing larger than a predetermined spatial spacing between the IDT pattern and the cap 14 itself.
The surface acoustic wave device has a limitation on its downsizing because the bonding wires 13 need a spatial spacing larger than a predetermined spatial spacing between the metal cap 14 and the IDT patterns. Therefore, instead of a configuration where the double-mode SAW filter (DMS) is connected with the terminals on the ceramic package side with the bonding wires 13, a configuration can be envisaged, where the double-mode SAW filter (DMS) is connected with the terminals on the ceramic package side with flip-chip connection.
FIG. 4 is a sectional view of an exemplary configuration envisaged when the double-mode SAW filter having the IDT patterns shown in FIG. 2 is mounted on a ceramic package with flip-chip connection.
Grounded terminals GND, an input terminal IN and an output terminal OUT are formed on the ceramic package 11 side.
The grounded terminals GND, the input terminal IN and the output terminal OUT of the IDT pattern on the IDT pattern side are each connected to the grounded terminals GND, the input terminal IN and the output terminal OUT on the ceramic package side such that each of those terminals corresponds to its own counterpart, with the side of the IDT patterns formed on the piezoelectric element 10 of the double-mode SAW filter (DMS) being set facing downward. Thereby, reduction of the area and the height for accommodating the double-mode SAW filter (DMS) in a ceramic package can be facilitated.
However, as shown in FIG. 4, when the double-mode SAW filter is accommodated in the ceramic package with flip-chip connection, degradation of the electrical characteristics of the SAW filter is recognized. FIGS. 5A and 5B are graphs showing and comparing the electrical characteristics of the double-mode SAW filter having the same configuration, that is, the double-mode SAW filter having the terminal IN (OUT) of the input (output) IDT1 and the terminal OUT (IN) connected commonly with the 2 (two) outputs (inputs) IDT2a and 2b arranged on the same side, accommodated in the ceramic package with wire bonding, with the electrical characteristics of the double-mode SAW filter accommodated in another ceramic package with flip-chip bonding.
That is, in FIGS. 5, the graph, FIG. 5A shows the pass-band characteristics of a receiving filter for guiding a received signal in a duplexer from an antenna to a receiving circuit. The graph, FIG. 5B shows the isolation of the duplexer from the transmitting side to the receiving side.
In general, to a receiving antenna filter in a PCS duplexer, an amount of attenuation equal to or more than 50 dB in a transmission band is required. 55 dB is required as the standard value for isolation in a transmission band.
As shown in FIG. 5A, a attenuation characteristics II outside the pass-bands from antenna to receiving side of the duplexer obtained when the filter is accommodated in a ceramic package with flip-chip bonding is degraded compared to an attenuation characteristics I in the case where the filter is accommodated with wire bonding, and has a region which does not satisfy the amount of attenuation equal to or more than 50 dB.
As shown in FIG. 5B, an isolation II from the transmission side to the receiving side of the duplexer obtained when the filter is accommodated in a ceramic package with flip-chip bonding has, compared to the case I with wire bonding, a region which does not satisfy the standard value of 55 dB for isolation.
The cause of the degradation of the electrical characteristics of the double-mode SAW filter in the case with flip-chip bonding is estimated that the spacing between the input terminal and the output terminal in the ceramic package is too narrow when the double-mode SAW filter is accommodated in the ceramic package with flip-chip bonding and, thereby, a capacity between the input terminal and the output terminal becomes large.
Based on the above estimation, the inventors measured the pass band characteristics of a double-mode SAW filter which has, as shown in FIG. 6, a wider spacing between the IN (OUT) terminal and the OUT (IN) terminal, and a GND pattern 4 formed in an area not limited to the surface layer and the internal layer of the ceramic package such that the GND pattern 4 corresponds to an area between the IN (OUT) terminal and the OUT (IN) terminal of the IDT pattern, compared to the IDT pattern in FIG. 1.
FIG. 7 is a diagram showing a terminal pattern on the ceramic package side corresponding to the IDT pattern of FIG. 6. The spacing between the IN (OUT) terminal and the OUT (IN) terminal is widened such that a grounding GND line (Line) can be formed in the spacing.
As described above, according to the configuration of FIG. 6, the IN (OUT) terminal and the OUT (IN) terminal are separated by the GND pattern 4 and, therefore, the corresponding coupling capacity formed between the input terminal and the output terminal can be reduced.
FIG. 8 is a graph of the pass band characteristics showing the effect of the reduction of the coupling capacity formed between the input terminal and the output terminal due to the above GND pattern 4.
As shown in FIG. 8, it can be understood that an amount of attenuation out of the pass band can be made larger in the direction of the arrows due to the reduction of the coupling capacity formed between the input terminal and the output terminal.
However, according to the above configuration, widening of the spacing between the input terminal and the output terminal is required in order to form the GND pattern 4 as shown in FIG. 7 and, because of this, the device configuration spreads out over a plane and, thereby, achieving downsizing is prevented.